Rebar And Fibermesh

 
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Old 09-22-2017, 01:54 PM   #1
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Rebar And Fibermesh


I'm having my house and shop slab foundations (monolithic) poured next week. 1700 sq/ft shop and 3000 sq/ft house & attached garage. I will have 3/8" rebar 2' on center. Concrete will be 5-1/2" thick. I asked the contractor about also using fibermesh and he said it just wasn't necessary. I don't want to piss money away but it seems like it would be good insurance, for a floor I am going to be looking at in both places for a long time. I hope to do some kind of finished concrete in both the house and shop. What are your thoughts about using both rebar and fibermesh?
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Old 09-22-2017, 02:10 PM   #2
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Re: Rebar And Fibermesh


Are you referring to the fiber added to the concrete mix, or welded wire mesh reinforcement?

Either way, with a rebar grid, you shouldn't need them. However, I personally would use #4 bar in a 5 1/2" slab. I would also probably go to an 18" or 16" grid pattern.

The fibers added to the mix don't really add any structural strength. They are mainly for crack control.


I also work and live in Northern California, a very active seismic zone, so I use steel in a way a lot of other folks don't.

The 1,300 SF basement for a 3 story house I'm building right now has 2 1/2 tons of steel in it.


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Old 09-22-2017, 02:17 PM   #3
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Re: Rebar And Fibermesh


I'm talking the fiber added to the to concrete mix. From what I've read, it seems like they both (steel & fiber) have separate purposes, but admittedly, I don't know all that much about concrete. I'm in Minnesota and this is a frost protected slab - no frost footings.
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Old 09-22-2017, 02:20 PM   #4
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Re: Rebar And Fibermesh


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The fibers added to the mix don't really add any structural strength. They are mainly for crack control.
Exactly. I hope to have an exposed concrete floor in the house and I don't want cracks, or at least I want to minimize them as much as possible.
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Old 09-22-2017, 02:31 PM   #5
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Re: Rebar And Fibermesh


Concrete with glass in it looks like it needs a haircut.


I've got a slab 30 x 42 right outside my office window poured in 1986 - still looks like it needs a haircut today.

And it cracked.

Only one question: why don't you trust your contractor's expertise?
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Old 09-22-2017, 02:51 PM   #6
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Re: Rebar And Fibermesh


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Exactly. I hope to have an exposed concrete floor in the house and I don't want cracks, or at least I want to minimize them as much as possible.

There's an old saying about concrete; "There are 2 types of concrete; Cracked concrete, and concrete that's going to crack."



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Old 09-22-2017, 07:20 PM   #7
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Re: Rebar And Fibermesh


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There's an old saying about concrete; "There are 2 types of concrete; Cracked concrete, and concrete that's going to crack."

Delta
Not necessarily, I did 3 separate 12'x24' slabs in my old shop with no control joints and none of them cracked after over 20 years. No rebar or glass, just WWF. But, I made sure everything below it was as good as it could be: consistently graded/tamped and 6" of 3/4" clean gravel, again, well tamped. IMO varying slab thickness,sloppy prep and too much H20 will guarantee cracking.
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Old 09-22-2017, 08:02 PM   #8
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Re: Rebar And Fibermesh


Either way, with a rebar grid, you shouldn't need them. However, I personally would use #4 bar in a 5 1/2" slab. I would also probably go to an 18" or 16" grid pattern.





Also,don't forget the placement of steel is important. Center of slab is close to useless,does little for compression or tension. Control joints should be an actual 25 % of slab thickness. Saw cutting should be done as soon as finishing is completed ,using an early entry saw.

Don't forget proper curing !
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Old 09-23-2017, 09:23 AM   #9
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Re: Rebar And Fibermesh


All good advice above. I also think % WC is highly important.
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Old 09-23-2017, 09:43 AM   #10
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Re: Rebar And Fibermesh


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Only one question: why don't you trust your contractor's expertise?
It's not that I don't trust him, it's that there are varying degrees of trust and everyone has a reason for their opinion, whether it be time, money, experience or a thousand other things, but it's hard to know which, and if it's in my best interest, theirs or both.

Just trying to gather as much info as I possibly can to make the best informed decision possible. That's why this forum is so cool.
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Old 09-23-2017, 09:54 AM   #11
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Re: Rebar And Fibermesh


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All good advice above. I also think % WC is highly important.
I read that over and over. How do I control/guard against that during the pour? I am the homeowner and the general, but I have hired a concrete contractor.
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Old 09-23-2017, 09:57 AM   #12
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Re: Rebar And Fibermesh


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Also,don't forget the placement of steel is important. Center of slab is close to useless,does little for compression or tension. Control joints should be an actual 25 % of slab thickness. Saw cutting should be done as soon as finishing is completed ,using an early entry saw.

Don't forget proper curing !
Where should it be if not center of slab? Top 1/3? Bottom 1/3? Or do you mean it should extend to the edge of the slab?

Control joints are schedule for the next morning, but I'll try to get them to do it before they leave. Hell it would save them a trip.
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Old 09-23-2017, 10:09 AM   #13
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Re: Rebar And Fibermesh


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Where should it be if not center of slab? Top 1/3? Bottom 1/3? Or do you mean it should extend to the edge of the slab?

Control joints are schedule for the next morning, but I'll try to get them to do it before they leave. Hell it would save them a trip.
Top 2" of slab will control cracking the best.

2" or so off the bottom where we put it for footings, etc., that might be subject to bending forces. If there are no significant loads on the floor, then top 1.5-2" is where I would put the grid.

But if there are footings in this slab, you'll have a grid, or just a couple bars, for the footings, and then a grid above for the slab.

I've saw cut control joints the day after. If fact, that's when I always do it. It might not be possible to do them the same day, it might be. Lots of variables in a pour.

If you've never worked with these guys before, it might be wise to check out some of their previous work. A concrete slab that you don't like is hard to fix, and what constitutes defective, from a professional trade perspective, like an ASTM standard, is a far cry from what you just don't like. Not that there is anything wrong with not liking it, but it'll be hard to convince anyone that there is a material defect in it.



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Old 09-23-2017, 11:05 AM   #14
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Re: Rebar And Fibermesh


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Originally Posted by climb View Post
I read that over and over. How do I control/guard against that during the pour? I am the homeowner and the general, but I have hired a concrete contractor.


If the WC stands for water content,it is very easy to control. Pick the slump you want for for design criteria lets not go over a 4. Have the batch plant mix it at a 3 and pay for plasticizer to bring it up to a 5. DOT approved batch plants will have all trucks supplied with a separate tank on truck with plasticizer. They arrive at your site,discharge some mix into wheelbarrow ,at a 3 it will stack in tub. Have them hit the magic button,the air shoots the "grease" into drum,rotate for 5 minutes and wallah out comes mix like you dumped a bunch of water in truck.

The beauty of plasticizer,it provides the work-ability you need without segregation of aggregate like water and no loss of strength from a change of water to cement ratio.
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Old 09-23-2017, 11:09 AM   #15
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Re: Rebar And Fibermesh


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Originally Posted by climb View Post
Where should it be if not center of slab? Top 1/3? Bottom 1/3? Or do you mean it should extend to the edge of the slab?

Control joints are schedule for the next morning, but I'll try to get them to do it before they leave. Hell it would save them a trip.


two inches off bottom would be sweet spot IMHO. Reason being,you are anticipating loading the slab,trucks,equipment etc. You want the steel to go into tension easily + 2" will give you the necessary coverage to protect it. By the way,what are you doing about a vapor barrier ?
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Old 09-23-2017, 11:35 AM   #16
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Re: Rebar And Fibermesh


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two inches off bottom would be sweet spot IMHO. Reason being,you are anticipating loading the slab,trucks,equipment etc. You want the steel to go into tension easily + 2" will give you the necessary coverage to protect it. By the way,what are you doing about a vapor barrier ?
House and the shop have the exact same slab detail. See pic. 4 #4 cage in footing - stirups every 2' and 3/8" 2' x 2' grid across the slab. "L" shaped house with attached garage, and rectangular detached shop. Deep compacted sand base, 4" of rock, poly, 2" foam insulation.

So I have one suggestion for the grid in the top 2" and one for bottom 2". Does it depend on the application/loads...house vs. shop?

Still would like to hear more opinions on fibermesh in addition to the rebar. What is the downside? Will it actually give me any benefit here?
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Last edited by climb; 09-23-2017 at 11:37 AM.
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Old 09-23-2017, 11:37 AM   #17
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Re: Rebar And Fibermesh


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It's not that I don't trust him,...
Quote:
How do I control/guard against that during the pour? I am the homeowner and the general, but I have hired a concrete contractor.
Yes it is all about trust. And you can ask all the damned questions you want, and get all the damned spiffy answers YOU WANT TO HEAR - but at some point, you are going to have to trust a contractor to perform work that cannot be evaluated by an answer you get from nameless faces on a forum.

See that quote above? What do you care about WC - you just dammit said you "hired a contractor".

Worse, you are asking advice not on how to do a job, but on how to tell if your contractor knows what he is talking about.

You & this thread belong in DIY.

gaaaack.
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Old 09-23-2017, 11:50 AM   #18
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Re: Rebar And Fibermesh


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Originally Posted by climb View Post
House and the shop have the exact same slab detail. See pic. 4 #4 cage in footing - stirups every 2' and 3/8" 2' x 2' grid across the slab. "L" shaped house with attached garage, and rectangular detached shop. Deep compacted sand base, 4" of rock, poly, 2" foam insulation.

So I have one suggestion for the grid in the top 2" and one for bottom 2". Does it depend on the application/loads...house vs. shop?

Still would like to hear more opinions on fibermesh in addition to the rebar. What is the downside? Will it actually give me any benefit here?



If you are considering 2 grids,one 2" from bottom,one 2" from top on a 5.50" slab,that only leaves 1.50" between them - steel thickness. That would create problems IMHO with 3/4 " aggregate. The key to good concrete is as some mentioned,very good base prep,sound placing and finishing procedures,control joints and proper curing.

The only down side to fiber is as STG mentioned,fuzzy slabs. In reality,the protruders can either be torched off or will wear away them self with traffic. Along with the added cost. If you feel you'd want fiber,go for it.
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Old 09-23-2017, 11:56 AM   #19
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Re: Rebar And Fibermesh


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If you are considering 2 grids,one 2" from bottom,one 2" from top on a 5.50" slab,that only leaves 1.50" between them - steel thickness. That would create problems IMHO with 3/4 " aggregate.
No just one 2' x 2' grid.
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Old 09-23-2017, 12:06 PM   #20
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Re: Rebar And Fibermesh


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The only down side to fiber is as STG mentioned,fuzzy slabs. In reality,the protruders can either be torched off or will wear away them self with traffic. Along with the added cost. If you feel you'd want fiber,go for it.

Exactly. I hope to have an exposed concrete floor in the house
no. Really, the sh!t is made out of glass. It doesn't burn off with a torch. I had the floor in one area power polished because the kids & the dogs got slivers of it in their bare feet.
Stuff stays there and gets infected.
I'm convinced the salt follows the fibers down and increases spalling too. And the trafficed areas just wear and expose more.

not nice.

glass was a fad around here. Don't see it any more.

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